Light enhancing films comprising elongated micro-prisms can be used to increase the light output into a limited field-of-view of a display; this increase is in comparison with the light output into those angles of the display without the light enhancing film. The enhancement is accomplished by redistributing light so that more light is included in that particular field-of-view through a process referred to a “recycling,” which is described more fully below.
The light enhancing films comprise an array of parallel ridges and grooves that form the elongated micro-prisms. Such light enhancing films are included in displays that additionally comprise a light source, a planar light guide optically coupled to the light source, and a transmissive spatial light modulator that receives light from the light guide. The spatial light modulator comprises an array of selectively activated pixels that can be used to form an image or pattern (e.g., text).
The light source may be disposed at an edge of the light guide to couple light into the light guide. Light guided within the light guide may be extracted from a front planar surface of the light guide using an array of extractor elements, for example, on a back planar surface of the light guide. The light enhancing film is disposed between the front planar surface of the light guide and the spatial light modulator such that the light enhancing film receives light from the front surface of the light guide. The plurality of micro-prisms in the light enhancing film are configured such that a portion of this light is transmitted through the light enhancing film into a limited range of angles. Another portion of the light is reflected back into the light guide. This reflected light may once again exit the light guide and may be transmitted through the light enhancing film into the limited range of angles or be reflected back by the light enhancing film once again. This process may be referred to as light recycling. Similarly the light enhancing films may be referred to as light recycling films, and the terms are used interchangeably herein. These terms are used even though the light enhancing film or light recycling film is not solely responsible for the recycling process or resultant enhancement. (For example, other components are used to redirect that light back to the light recycling film to enable the recycling process and produce the enhancement.) Advantageously, through this light recycling process, the light recycling film increases the light directed into the limited range of angles, increasing luminance for those directions, and reduces the luminance in other directions.
One drawback of conventional light recycling films is that periodicity introduced by the light recycling films contribute to the formation of Moiré fringes. Moiré fringes result from the superposition of a plurality of periodic structures. In this case, periodicity is introduced by the array of parallel ridges and grooves in the light recycling film and the extractors used to couple the light out of the light guide. Additionally, Moiré fringes can occur because of periodicity with the pixel in the spatial light modulator.
What is needed, therefore, is a way to reduce or eliminate the contributions to the Moiré effect.